Altimeter Calibration Height

"Jose" wrote in message
. ..

No. The altimeter senses pressure at the level of the instrument itself.
It indicates altitude depending on its calibration and kollsman setting.


You're contradicting yourself.

The static lines are vented to the atmosphere, so the change in pressure
with altitude takes place in the static lines just as it does in the
atmosphere.

That's not logic, that's physics. Many things that are logical (heavy
objects fall faster) are not supported by the laws of physics. That
degree of physics knowledge is not necessary to piloting an aircraft
(though it is helpful), thus I expect not all pilots know this.

Jose
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"Jose" wrote in message
...

Many things that are logical (heavy objects fall faster) are not supported
by the laws of physics.

That's not logical.

No. The altimeter senses pressure at the level of the instrument itself.
It indicates altitude depending on its calibration and kollsman setting.
You're contradicting yourself.

Only if pressure and altitude are the same.

And only if sensing and indicating are the same.

Jose
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That's not logical.

Of course it's logical. The heavier they are, the more force is exerted
on them. You can test this by dropping a stone and a feather.

As it turns out, in our universe, increased inertia exactly
counterbalances the increased gravitational force, but there is no
(logical) reason for the universe to operate that way. In fact, I am
not convinced that it does operate that way to the last decimal point.

If logic were sufficient, there would be no experimental physics.

Jose
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Get high on gasoline: fly an airplane.
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Huh? Drag force isn't logical.
It is to me.

No, it is merely consistant with your experience. That is empirical,
not theoretical. Drag force can be derived in theory, but that requires
knowledge of atoms and molecules, which themselves are not logical. The
universe could be made up of wiggly strings, for all we know. That it
is made up of atoms is another =experimental= result.

That they are governed by quantum mechanics is yet =another=
experimental result, which is most =definately= not logical.

Jose
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Get high on gasoline: fly an airplane.
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Recently, Steven P. McNicoll posted:

"Jose" wrote in message
t...

Poor logic. An altimeter indicates whatever it is set to indicate.
It senses pressure, and moves hands on a dial. Take the case where
you are in a one hundred foot tall aircraft with the altimeter right
at the top of the cockpit with you, a hundred feet in the air.
(We'll neglect the tail for now).

If you don't get an altimeter setting, you will (likely) set the
altimeter so that the hands indicate the airport elevation as
indicated on your charts, even though you and the instrument are a
hundred feet higher.

If you do get an altimeter setting, you'll set it for that. Then the
question becomes (since the altimeter doesn't know that it's in a
tall airplane) whether, at calibration in the shop, it was set to
indicate actual instrument altitude or to indicate something else
(like instrument altitude minus a hundred feet). I bet there are
standards for that. I don't know what they are, but they may well
incorporate the hundred feet deviation, since it is the
=installation= that is certified.

Also, even if the static port and the instrument themselves are
separated in altitude, it would be the instrument altitude's
pressure that is sensed, since the "column of air" is connected, and
ends at the instrument.


In other words, an altimeter indicates altitude at the level of the
instrument itself.

Perhaps you misunderstood Jose's example, but I believe he's correct;
while an altimeter senses the pressure at the level of the insturment, it
*indicates* the calibration set by either the shop (e.g. compensating for
the altitude of the installation) or the pilot via the Kollsman window.

Neil

"Jose" wrote in message
t...

Of course it's logical. The heavier they are, the more force is exerted
on them. You can test this by dropping a stone and a feather.


I'd have to do it in a vacuum to eliminate the drag force.

"Neil Gould" wrote in message
et...

Perhaps you misunderstood Jose's example, but I believe he's correct;
while an altimeter senses the pressure at the level of the insturment, it
*indicates* the calibration set by either the shop (e.g. compensating for
the altitude of the installation) or the pilot via the Kollsman window.


I understood it, I didn't say he was wrong.

I agree that it measures pressure and that the indicated altitude is based
on calibration. I agree that it could be calibrated for the bottom, mid, or
top of the plane (for whatever it would matter). Personally, my altimeter
was calibrated sitting on a bench with no regard for the difference in
height between the landing gear, the static port, or the position of the
altimeter.

From a purely theoretical view, I don't think you can say that an altimeter
measures pressure at the location of the instrument. It's a closed system to
the static port. While the pressure inside the tubing will respond to
elevation changes similar to the open atmosphere, there is a difference.
Admitedly, the difference over the height of an aircraft is insignificant.
But stretch that tubing to a low pressure system in the next state and your
altimeter will defininetly not be reading the local pressure.

The question remains, are big airplane altimeters calibrated to account for
the distance between the landing gear and the instrument. Someone else asked
about when setting the altimeter based on field elevation rather than by
Kollsman setting. Another way to ask the question is, "When setting the
altimeter to field elevation in a tall airliner, how close is the Kollsman
value to the local barametric pressure?"

--
-------------------------------
Travis
Lake N3094P
PWK
"Jose" wrote in message
. ..
In other words, an altimeter indicates altitude at the level of the
instrument itself.

No. The altimeter senses pressure at the level of the instrument itself.
It indicates altitude depending on its calibration and kollsman setting.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.